The invention relates to a vacuum supply system for a brake power assist unit of a vehicle, as well as for at least one additional consuming device, and having a vacuum generating device connected to a main pipe being provided with a first check valve between a suction connection of the vacuum generating device and the brake power assist unit. A branch pipe branches off from the main pipe between the suction connection and the first check valve and leads to the additional consuming devices. A second check valve in the branch pipe lets through a suction flow from the additional consuming device to the vacuum generating device, and a flow control device, fluidly connected in series with the second check valve is also provided.
A vacuum supply system of this type is known from German Unexamined Patent Application No. DE-OS 31 24 153.
Check valves between vacuum generating devices and different consuming devices, or groups of consuming devices, are normally installed in the corresponding supply pipes in order to avoid incoming atmospheric air, due to leakages at individual consuming devices, which has a disadvantageous effect on the vacuum level at other consuming devices which are not leaking.
Flow control devices such as brake power assist units which are arranged in front of secondary or so-called vacuum-consuming comfort devices (e.g., headlight adjusting motors, air-conditioning system control motors, etc.) require vacuum supply priority, which is desirable for reasons of driving safety. With a secure priority system, such priority devices can be evacuated by the same vacuum generating device as the secondary vacuum consuming devices.
It is known to use vacuum pumps with two separate suction connections (German Unexamined Patent Application No. DE-OS 32 43 984), in which case the suction connection belonging to the secondary consuming devices is equipped with a flow control device.
The above-mentioned arrangement is in use in diesel engine equipped vehicles manufactured by Daimler-Benz Aktiengesellschaft, in which however, a check valve is arranged in the suction pipe leading from the brake power assist unit to the vacuum source in order to maintain the vacuum level at the brake power assist unit during an atmospheric pressure flow short circuit of the vacuum pump. Between the brake power assist unit and the check valve, a throttled secondary pipe branches off from the suction pipe leading to the brake power assist unit. This throttled secondary pipe is bled only for the zero adjustment of the injection pump control rod when the diesel engine is shut off. For safety reasons, it must be a single pipe.
The growing number of comfort enhancing vacuum consuming devices used in motor vehicles make it necessary to be able to continuously ensure their vacuum supply. This supply should be improved over known systems mainly in view of the requirement that the lowest vacuum level be available at the brake power assist unit only when the power brake is actually actuated.
It is therefore the object of the invention to develop a vacuum supply system of this type, such that when the power brake is not actuated, the vacuum supply of secondary consuming devices is improved.
According to the invention, this object is achieved by having the flow line connected to the vacuum consuming device have a bypass pipe with a second flow control device fluidly arranged in parallel to both the second check valve and the first flow control device, which is itself connected in series with the second check valve.
By a bridging provided by the bypass, the check valve flow control device unit connected between the vacuum generating device and the secondary consuming devices, ensures that overall flow resistance of the supply of the secondary consuming devices is reduced, despite the additional flow control devices, so that when the power brake is not actuated, practically the complete vacuum output is available for use by the secondary consuming devices.
A vacuum supply system for a brake power assist unit and at least one additional consuming device is known from German Unexamined Patent Application No. DE-OS 31 05 041. There, the branch pipe of the secondary consuming device contains a flow control device for securing the priority of the supply of the brake power assist unit as well as a check valve, which fluidly is switched in parallel with this flow control device. Although the throttled pipe section is called a "detour pipe", it actually, because of the provided orientation of the parallel check valve through which an evacuation of the secondary consuming device must not be possible, represents the only bleeding possibility of the secondary consuming device. The check valve permits only an additional evacuation of the brake power assist unit at the expense of the vacuum level at the secondary consuming device.
Because of the specified opposite objectives, the mentioned German Unexamined Patent Application No. DE-OS 31 05 041 can supply no reference with respect to the construction of the type of vacuum supply system which has the flow line connected to the vacuum consuming device equipped with a bypass pipe with a second flow control device fluidly arranged in parallel to both the second check valve and the first flow control device, which is itself connected in series with the second check valve.
It is advantageous if the bypass pipe is connected to its own suction connection at the vacuum generating device which is constructed as a vacuum pump having two separate suction connections. Further, the bypass pipe and the second flow control device can be arranged fluidly in parallel with this second check valve and the first flow control device and can be integrated in a common housing having two pipe connections to the branch pipe. Alternatively, the two check valves, the first flow control device and the junction of the branch pipe from the main pipe can be arranged in a common housing having three pipe connections.
It is desirable that the second check valve be prestressed into its shut position with a higher restoring force than the first check valve, as well as having the first flow control device be arranged between the second check valve and the additional consuming device.
For control purposes, the second check valve can be shut by an outside power source. The outside power source can be controlled by an electric switch which is coupled with an actuating arrangement of the brake power assist unit. This actuating element could be mechanical or an electric pressure switch acted upon by fluidic brake system pressure. Alternatively, the electric switch could be actuated by the stop light switch of the motor vehicle. The second check valve is shut when the absolute pressure in front of the first check valve is higher than approximately 0.4 bar.
The flow control device arranged in the bypass pipe can have its smallest clear cross-section be of a diameter of between 0.6 and 1.0 mm and preferably 0.8 mm. The vacuum compensating tank, which fluidly is assigned to the at least one consuming device is connected to the bypass pipe.
Special constructions of check valves combined with flow control devices are known in the vacuum supply systems of the above-mentioned types. However, in these, a flow control device becomes operative only when the check valve is opened against its restoring force. German Unexamined Patent Application No. DE-OS 28 14 384 describes a check valve having a flow control device which is integrated and fluidly is arranged in parallel.
By providing the check valve connected between the vacuum generating device and the consuming device(s) with a relatively strong return spring, it is ensured that the secondary consuming devices and the relatively wide flow control device, which is connected behind it (in parallel to the narrow bypass flow control device) will not be evacuated by means of this check valve before a required vacuum level is reached at the brake power assist unit. The priority of the supply of the brake power assist unit continues to be ensured, because as soon as its vacuum chamber is bled during braking operations, the (absolute) pressure in the suction pipe will rise, and the second check valve will close. A slight increase of the achievable (absolute) vacuum level as a result of the bypass pipe, while the consuming device output remains the same, can be accepted without any difficulties.
Another advantageous development for even more reliably ensuring the priority of the brake power assist unit from the vacuum supply system is achieved, if the second check valve, during each braking operation, can be power actuated by any of electrical, hydraulic or pneumatic means.
A check valve in a vacuum supply system for a brake power assist unit which can be power operated is known from German Patent No. DE-AS 11 20 293. In addition to being operated by pressure differences, it is also controlled by an electric switch coupled with the brake pedal. It is opened electromagnetically and connects the brake power assist unit with a vacuum compensating tank, in order to improve the vacuum level acting upon it.
According to another embodiment of the invention (shown in FIG. 4), a valve is arranged in a secondary pipe between the vacuum generating device and the at least one secondary consuming device and can be controlled as a function of the instantaneously existing vacuum and can be switched into at least two positions.
The secondary pipe either starts out directly from the vacuum generating device, or from its own suction connection (dual-flow system); or leads only indirectly to the secondary consuming device (single-flow system); i.e., leading away as a branch pipe, in front of the check valve, from the main pipe leading to the brake power assist unit.
The overall throttling of the secondary consuming device suction flow is varied, in that in one position of the valve an additional flow control device can be connected in series with the already existing flow control device. In the other valve position, no additional throttling takes place. The valve switches over with a given vacuum value range, i.e., at approximately 0.5 bar. Above this range (in the direction of the atmospheric pressure), the suction flow is throttled more, in order to continue to evacuate the brake power assist unit as a priority. Below this range, the vacuum level existing in the system is sufficient for a reliable operation of the brake power assist unit, so that the throttling of the secondary consuming device suction flow may be reduced without any loss of reliability, and thus the supply of the secondary consuming devices may be improved.
With respect to their basic principles, valve arrangements which allow a variable throttling of flows are known. German Unexamined Patent Application No. DE-OS 30 11 360 shows a compressed air system for a vehicle, which has a 3/2-way valve, which pneumatically can be switched into two positions and to two corresponding pipe sections. In one position, this 3/2-way valve switches a freely passable pipe section into a compressed air pipe and in the other position, switches a pipe section equipped with a control device into a compressed air pipe. By this arrangement, the amount of air is reduced which must be released by a pressure regulator connected behind the valve since the system pressure behind the valve, or in the pressure regulator, switches it, starting from a certain sufficient pressure level into a "throttling" position. In addition, from German Patent No. DE-PS 35 26 763, a similar arrangement of an electromagnetically switchable 3/2-way valve is known which, as a function of an electrically detected system pressure, switches either a throttled or an unthrottled pipe section between a bi-pressure pump and consuming devices. This arrangement has the purpose of making it possible to connect several consuming devices with different pressure level requirements to a single pressure source. However, neither of the latter texts supplies any reference to the endeavored improvement referenced in this invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.